The present invention relates to a power supply circuit.
Power supply circuits including various structures have been proposed conventionally (see Patent document 1 below, for instance).
For instance, a so-called DC-DC converter is usually made up of a chopper circuit that converts a direct current into a pulse waveform, and an LC circuit (circuit including inductance element and capacitor) that rectifies the pulse waveform. Further, the DC-DC converter adjusts its output voltage to a target value by adjusting a duty ratio of the pulse waveform.
However, the DC-DC converter including the structure as described above has a problem that it may not work normally unless an input voltage is higher than the target value of the output voltage by a predetermined limit value or more, depending on a voltage drop due to a switching device, other resistance components, or the like included in the chopper circuit, a conversion characteristic between the pulse waveform and the direct current, or the like.
Therefore, when a battery is used as a power source of the input voltage, for instance, in spite of the sufficient output of the battery, a case where the battery cannot be used may occur when the voltage is decreased below the predetermined limit value, because of the relationship between the voltage and the target value of the output voltage of the DC-DC converter. Therefore, there is a case where the battery should be replaced with a new one in a period of time shorter than a rated life of the battery.    [Patent document 1] Japanese Patent Application Laid-open No. 2001-195136